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[NavigationTiming2] Comments and questions about the Navigation Timing 2 draft

From: McCall, Mike <mmccall@akamai.com>
Date: Tue, 9 Oct 2012 15:22:41 -0400
To: public-web-perf <public-web-perf@w3.org>
Message-ID: <CC99F1C1.E081%mmccall@akamai.com>
Hi web-perf,

I just wanted to give some feedback on the Navigation Timing 2 draft.
After reading the spec, it appears that a big change has been made to the
interface and how it reports timestamps.  In particular, it looks like
navigationStart will always be 0.000, and everything will be reported as a
delta from that time.  If this is in fact, correct, then I am concerned
that by not having an absolute timestamp available, it will be difficult
to correlate Navigation Timing measurements with backend logs, which can
be useful for postmortem debugging.  Perhaps an additional attribute could
be added, 'navigationStartTimeStamp' for example, which would contain the
epoch time of navigationStart.  To be clear, I don't think we need an
epoch timestamp for every attribute, but knowing the timestamp from which
the measurements are based would be useful.

Also, I noticed that the concept of first paint/pixel is omitted in this
draft.  While IE supports it with a vendor prefix, and Chrome exposes it
through their window.chrome.loadTimes() object (not sure if other browsers
expose it), it would be good if it were included as part of the official
spec so there was a browser-independent place to gather this data.  Some
3rd-party testing vendors including webpagetest.org, report some concept
of the time the browser began rendering the page, and I think it would be
valuable here too.

Finally, based only on the spec, it seems difficult to determine whether
or not the document was retrieved from cache or not.  Is it possible to
determine that a document was not downloaded from the server, but
retrieved from cache, via the Navigation Timing API?  If not, would a
'fetchType' attribute be useful here?

Mike
Received on Wednesday, 10 October 2012 15:47:49 GMT

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